Monthly Archives: March 2017

Finally, the last accessory I ordered for my Raspberry Pi Zero (that’s the little red thing behind my keyboard) has arrived – an Acer 43″ ET430K monitor. The Zero won’t quite drive this monitor at its maximum resolution of 3840×2160 pixels, but as you can see, you get enough real estate to do real graphics with “ASCII Art” (well, Unicode Art, anyway)!

Some readers will recognise the image as the famous Mandelbrot Set, named after the mathematician Benoit Mandelbrot, who studied fractals. According to Wikipedia a fractal is a mathematical set that exhibits a repeating pattern displayed at every scale – they are interesting because they produce patterns that are very similar to things we can see around us – both in living organisms and landscapes – they seem to be part of the fundamental fabric of the universe.

Fractals are are also interesting because they produce images of staggering beauty, as you can see on the pages linked to above and one of our rotating banner pages, where a one-line form of the APL expression which produced the image is embedded:

The colourful images of the Mandelbrot set are produced by looking at a selection of points in the complex plane. For each point c, start with a value of 0 for z, repeat the computation z = c + z², and colour the point according to the number of iterations required before the function becomes “unbounded in value”.

In APL, the function for each iteration can be expressed as {c+⍵*2}, where c is the point and ⍵ (the right argument of the function) is z (initially 0, and subsequently the result of the previous iteration):

If you are not familiar with complex numbers, those results may look a bit odd. While complex addition just requires adding the real and the imaginary parts independently, the result of multiplication of two complex numbers (a + bi) and (c + di) is defined as (ac-db) + (bc+ad)i. Geometrically, complex multiplication is a combined stretching and rotation operation.

Typing all those f’s gets a bit tedious, fortunately APL has a power operator ⍣, a second-order function that can be used to repeatedly apply a function. We can also compute the magnitude of the result number using |:

(|(f⍣6)0)(|(f⍣7)0)
8.853E7 7.837E15

Let’s take a look at what happens if we choose a point inside the Mandelbrot set:

Above, I used an anonymous function so I could pass the number of iterations in as a parameter, and use the each operator ¨ to generate all the results up to ten iterations. In this case, we can see that the magnitude of the result stabilises, which is why the point 0.1 + 0.1i is considered to be inside the Mandelbrot set.

Points which are just outside the set will require varying numbers of applications of f before the magnitude “explodes”, and if you colour points according to how many iterations are needed and pick interesting areas along the edge of the set, great beauty is revealed.

Our next task is to create array of points in the complex plane. A helper function unitstep generates values between zero and 1 with a desired number of steps, so I can vary the resolution and size of the image. Using it, I can create two ranges, multiply one of them by i (0J1 in APL) and use an addition table (∘.+) to generate the array:

The result is subtracted from 0.7J0.5 (0.7 + 0.5i) to get the origin of the complex plane slightly off centre in the middle, and multiplied the whole thing by 3 to get a set of values that brackets the Mandelbrot set.

Note that APL expressions are typically rank and shape invariant, so our function f can be applied to the entire array without changes. Since our goal is only to produce ASCII art, we don’t need to count iterations, we can just compare the magnitude of the result with 9 to decide whether the point has escaped:

We can see that points around the edge start escaping after 3 iterations. Using an anonymous function again, we can observe more and more points escape, until we recognise the (very low resolution) outline of the Mandelbrot set:

The last example allowed me to sneak in a preview of the new “at” operator coming in Dyalog v16.0. It is a functional merge operator that I am using to insert the formatted right argument (the number of iterations) into position (0 0) of each matrix.

If I use our remote IDE (RIDE) on my Windows machine and connect to the Pi, I can have an APL session on the Pi with 3840×2160 resolution. In this example, I experimented with grey scale “colouring” the result by rounding down and capping the result at 10, and using the character ⍟ (darkest) for 0, ○ for 1-5, × for 6-9, and blank for points that “escaped”, by indexing into an array of ten characters:

Retro is back in style – but of course I should point out that we can produce “proper” graphics using SharpPlot, a cross-platform graphics package that is included with Dyalog APL on all platforms. I’ve enhanced the ReactionTimer namespace with a function called SPHistogram. If you are using RIDE as the front end to APL on the Pi, this will render output from SharpPlot in a window:

ReactionTimer.SPHistogram times

The original ASCII chart simply plotted the observations in the order that they occurred. Above, the shaded areas show how many observations there were in each bucket (2 between 250ms and 275ms, 3 between 275ms and 300ms, and so on). At the same time, the individual observations can be seen along the X axis as vertical red lines. It is a little unfortunate that, presumably due to some artifact of the timing process, the values 251 and 294 occur twice – and these lines are drawn on top of each other.

The example highlights one of the things that makes SharpPlot a bit special: We have overlaid a histogram showing the frequency of reactions in each bucket, and used a “scatterplot” to where the Y value is always 0, to mark the individual observations along the X axis.

Hopefully the code is more or less self-explanatory, but if you’d like to learn more about SharpPlot there is excellent documentation at http://sharpplot.com. The documentation is actually written for C# users, but there is an illustration of how to translate the documentation to APL (and VB) at http://www.sharpplot.com/Languages.htm.

I have a bit of a cold today, so I decided that instead of hopping in an icy car and driving to the office in order to spend the day drinking coffee and answering e-mail, I should stay at home, turn up the radiators, make lots of tea (with honey!) and have some fun writing code on my Raspberry Pi! Can there be a better way to ensure a speedy recovery?

By the way, if you are already a user of Dyalog APL but you haven’t got a Pi yet, you should read The APLer’s Quick-start Guide to the Raspberry Pi, which Romilly Cocking completed a few days ago. This explains what your options are for buying hardware, and how to get going with a free copy of Dyalog APL. If you don’t read it now, your cold may be over before all the bits are delivered, so hurry up!

I wasn’t feeling THAT energetic this morning, so I decided to ease back into things by trying to replicate Romilly’s reaction timer, which is written in MicroPython. To begin with, I extended the microbit class that I developed for the Morse code display function with a few new methods to cover the API calls that allow me to check the state of the two buttons on each side of the micro:bit display (see the image above).

First, I added a method called is_true, which takes a MicroPython expression as an argument, evaluates it using the PyREPL function, and returns 0 or 1 depending on whether the expression returns False or True (and fails if it returns something else):

This is a useful building block, which allows the simple construction of functions like was_pressed, which takes ‘a’ or ‘b’ as an argument and tells you whether the button in question has been pressed since you last asked:

Once this is done, writing the Play function in the ReactionTimer namespace is easy, you can find the rest of the code on GitHub. The APL version is different from the MicroPython version in that – once the user presses button B and stops the game – the reaction times (in milliseconds) are returned as an integer vector. So now we can have some fun with data!

In the spirit of the micro:bit, I thought I’d produce a low tech character based chart, trying to remember the skills that were start-of-the-art when I started using APL. The AsciiChart function in the ReactionTimer namespace takes a vector of timings as the right argument, and produces a chart:

The code (which is also on GitHub) is listed below. Because this is a pure function, I used the modern dfns style of function definition rather than the old style procedural form that I’ve used for the object oriented code. The function works by allocating each value to a bucket of the size defined by the variable scale. The fun part is the use of the outer product with equals (∘.=) between the list of buckets on the left (rb) and the scaled values on the right (⌊⍵÷scale) – and then using this Boolean array to index into a two-element character vector to produce the chart. The rest is scaling calculations and finally decorating with “tik marks”: